1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) display device and a thermal transfer donor film used in manufacturing an organic EL display device. More particularly, the invention relates to an EL display device having a high efficiency of light extraction from an organic light-emitting portion. The higher efficiency is caused in part by a photonic crystal layer located directly on a stack formed on the substrate. Additionally, a laser induced thermal transfer donor film for the EL display device is used to form the photonic crystal layer on the stack.
2. Description of the Related Art
An electroluminescent (EL) display device forms viewable images by reflecting or shining light through an organic thin film material (e.g., a light emitting portion) sandwiched between millions of anodes and cathodes that are formed on opposing surfaces of two parallel glass substrates. Applying a voltage difference to each anode/cathode pair (e.g., pixel) alters the physical properties of the organic light emitting layer. When the voltage differences are applied in discrete amounts, various shades of colors are produced. Organic EL display devices are popular because they are driven by low voltages, are light and thin, and offer wide viewing angles and fast response times.
As mentioned above, the light-emitting portion of the EL display device includes an anode, a light-emitting layer, and a cathode sequentially formed on each other. The light-emitting layer may include an emitting layer (EML) in which exitons are formed by the recombination of holes and electrons to create light. An exiton is an electrically neutral excited state of an insulator or semiconductor, often regarded as a bound state of an electron and an electron hole (“hole”). A hole is a vacant position left in a crystal by the absence of an electron. The EML may further include: an electron transport layer (ETL) located between the cathode and an emitting layer to transport holes and electrons more smoothly to the emitting layer thereby increasing emitting efficiency; a hole transport layer (HTL) located between the anode and the emitting layer; a hole injection layer (HIL) located between the anode and the hole transportation layer; and an electron injection layer (EIL) located between the cathode and the electron transportation layer. Exemplary conventional light-emitting layers can be composed of copper phthalocyanine (CuPc), N,N′-Di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB), or tris-8-hydroxyquinoline aluminum (Alq3).
The light efficiency (e.g., the efficiency at which light is emitted) of such a light-emitting portion depends on internal efficiency, and the efficiency of other layers of the EL display device (external efficiency). A layer's internal efficiency varies depending on the photoelectric conversion efficiency of the material of which the organic light-emitting portion is composed. Similarly, external efficiency varies depending on the refractive index of each layer of the organic EL display device. The external efficiency is also called light coupling efficiency. A problem is that external efficiency is reduced when light emitted from the organic light-emitting layer has an outgoing angle greater than a critical angle of one of the layers. When this happens, reflection occurs at the surface of the layer. Reflection reduces the light, and causes it to emit externally.
Table 1 illustratively shows the light coupling efficiency of a transparent substrate formed of glass and an electrode layer formed of indium-tin-oxide (ITO), for each of blue (B), red (R), and green (G) light. The light coupling efficiency is calculated from the refractive index of each layer, and Nin and Nout indicate the refractive index of the layer where the light enters and emits, respectively.
TABLE 1Blue EmittingRed EmittingGreen EmittingLayerLayerLayerWave Length (nm)450620530Electrode Layer2.011.761.93Refractive Index (N)Substrate Refractive1.5251.5151.52Index (N)Light Coupling29%37%34%Efficiency
It can be seen from Table 1 that the light generated from each emitting layer may be reduced by more than 60% due to the refractive index difference between the electrode layer and the substrate. Various methods have been presented to increase such light coupling efficiency.
For example, the Japanese Patent Publication Gazette No. Hei 11-283751 discloses a structure that includes a diffraction grating or zone plate formed on a substrate. This reference also discloses diffracted light leaving an organic film and an Indium Tin Oxide (ITO) electrode.
In such an organic EL device, since irregularities occur on a surface of a substrate, a fine electrode pattern layer, or a separate diffraction grating must be included. This requirement complicates the manufacturing process, making it difficult to attain efficient productivity. Also, the formation of an organic layer on the irregularities in the surface of the substrate or the fine electrode pattern layer increases the overall roughness of the organic layer, which increases current leakage. Current leakage, in turn, deteriorates the durability and reliability of the organic EL device.
An organic EL display device preventing a decrease of light coupling efficiency is disclosed in the Japanese Patent Publication Gazette No. Sho 63-17269. The disclosed organic EL display device includes a substrate having light condensers, such as projecting lenses.
Another organic EL display device is disclosed in the Japanese Patent Publication Gazette No. Hei 1-29394. The display includes a first dielectric layer interposed between a transparent electrode layer and an emitting layer. Additionally, a second dielectric layer having a refractive index less than that of the first dielectric layer and greater than that of the transparent electrode layer is also disclosed.
FIG. 1 is a partial cross-sectional view of a conventional organic EL display device. As shown, an organic light emitting portion including two electrode layers 21 and 22 is formed on a substrate (not shown), and a sealing substrate 10 is formed on a photonic crystal layer 41. A spatial layer 40 formed between the photonic crystal layer 41 and an organic light-emitting portion is either a vacuum or is filled with an inert gas.
Use of the photonic crystal layer 41 may increase light coupling efficiency, however the path along which light travels must be structurally even. Otherwise screen display quality degrades. In order to achieve a uniform screen display quality, the spatial layers 40 should be regularly spaced in the regions where the light of an organic EL display device travels. However, such constraints limit the design and manufacture of EL display devices. Such problems also apply to active matrix (AM) organic EL display devices.